linux-kernel - Re: [RFC PATCH 0/4] Enable Clang's Source-based Code Coverage and MC/DC for x86-64

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Message-ID: <20251015073701.GZ3419281@noisy.programming.kicks-ass.net>
Date: Wed, 15 Oct 2025 09:37:01 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sasha Levin <sashal@...nel.org>
Cc: nathan@...nel.org, Matt.Kelly2@...ing.com, akpm@...ux-foundation.org,
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	wentaoz5@...inois.edu, x86@...nel.org
Subject: Re: [RFC PATCH 0/4] Enable Clang's Source-based Code Coverage and
 MC/DC for x86-64

On Tue, Oct 14, 2025 at 07:26:35PM -0400, Sasha Levin wrote:
> This series adds support for Clang's Source-based Code Coverage to the
> Linux kernel, enabling more accurate coverage measurement at the source
> level compared to gcov. This is particularly valuable for safety-critical
> use cases (automotive, medical, aerospace) where MC/DC coverage is required
> for certification.
> 
> Changes since previous patchset [1]:
> - Rebased on v6.18-rc1
> - Adapted to lib/crypto reorganization (curve25519 exclusion moved to
>   lib/crypto/Makefile)
> - Minor correctness fixes throughout the codebase
> 
> The implementation has been tested with a kernel build using Clang 18+ and
> boots successfully in a KVM environment with instrumentation enabled.
> 
> [1] https://lore.kernel.org/all/20240905043245.1389509-1-wentaoz5@illinois.edu/
> 
> Wentao Zhang (4):
>   llvm-cov: add Clang's Source-based Code Coverage support
>   llvm-cov: add Clang's MC/DC support
>   x86: disable llvm-cov instrumentation
>   x86: enable llvm-cov support
> 
>  Makefile                          |   9 ++
>  arch/Kconfig                      |   1 +
>  arch/x86/Kconfig                  |   2 +
>  arch/x86/crypto/Makefile          |   1 +
>  arch/x86/kernel/vmlinux.lds.S     |   2 +
>  include/asm-generic/vmlinux.lds.h |  36 +++++
>  kernel/Makefile                   |   1 +
>  kernel/llvm-cov/Kconfig           | 121 ++++++++++++++
>  kernel/llvm-cov/Makefile          |   8 +
>  kernel/llvm-cov/fs.c              | 253 ++++++++++++++++++++++++++++++
>  kernel/llvm-cov/llvm-cov.h        | 157 ++++++++++++++++++
>  lib/crypto/Makefile               |   3 +-
>  scripts/Makefile.lib              |  23 +++
>  scripts/mod/modpost.c             |   2 +

I'm thinking I'm going to NAK this based on the fact that I'm not
interested in playing file based games. As long as this thing doesn't
honour noinstr I don't want this near x86.

And we have kcov support, and gcov and now llvm-cov, surely 3 coverage
solutions is like 2 too many?